The Manti, Utah, landslide

Abstract

PART A: The Manti landslide is in Manti Canyon on the west side of the Wasatch Plateau in central Utah. In early June 1974, coincident with the melting of a snowpack, a rock slump/debris flow occurred on the south rim of Manti Canyon. Part of the slumped material mixed with meltwater and mobilized into a series of debris flows that traveled down the slope a distance of as much as 1.2 km. Most of the flows were deposited either at the base of the steep rocks of the canyon rim or at the site of an old, silted reservoir. A small part of the debris flow deposit stopped on the head of the very large, relatively inactive Manti landslide. The upper part of the landslide began moving as cracks propagated downslope. A little more than a year later, August 1975, movement extended the full length of the old landslide, and about 19 million m 3 of debris about 3 km long and as much as 800 m wide threatened to block the canyon. The upper part of the landslide apparently had moved small amounts between 1939 and 1974. This part of the landslide, identifiable on pre-1974 aerial photographs, consisted of well-defined linears on the landslide flanks and two large internal toe bulges about 2 km downslope from the head. The abrupt reactivation in 1974 proceeded quickly after the debris flows had provided a surcharge in the head and crown area. Movement propagated downslope at 4-5 m/h for the first few days following reactivation. During 1974, the reactivation probably encompassed all the parts of the landslide that had moved small amounts between 1939 and 1974. Movement nearly or completely stopped during the winter of 1974-75, but began again in the spring of 1975. The landslide enlarged from the flanks of the internal toe bulges to Manti Creek at a rate of 2-3 m/h. Movement stopped again during the winter of 1975-76 and began again in the spring of 1976. Thereafter, the displacements have been small compared to earlier. The displacement rates for the landslide were variable depending on where and when they were measured. At the waterline crossing, about 500 m downslope from the head, the maximum rate was about 1 mid, and the peak rate occurred within the first 30 days following reactivation. Almost a year later, during the spring of 1975, the rates there were 0.1-0.3 mid. By the time movement had propagated 2.5 km downslope to Manti Creek, more than 40 m of displacement had occurred at the waterline. Cracks were first noted at Manti Creek on June 21,1975, but movement was initially very slow there. The maximum rate of sliding of about 3.1 mid occurred during the period September 1-19,1975, and the movement decreased and stopped during the winter of 1975-76. At the time the lower part of the landslide was moving rapidly, the rate farther upslope was small. The result was that the landslide changed from being in compression, which was caused by loading from above, to being in extension, which was caused by the lower part moving faster than the upper part. One of the results of the extension was that the landslide pulled apart on a steeper segment of the slope and exposed the failure surface. Since the fall of 1975, the landslide has been separated into two independent parts. There has been no movement in the lower part, whereas movements in the upper part have continued at a rate of a few meters per year. Although the landslide is 6.5 km from the nearest permanent dwelling, it cost nearly $2 million in actual damages and in preparation for a major disaster that did not occur. The waterline for the city of Manti was replaced and a well was drilled to provide an emergency water supply. PART B: The Manti landslide abuts, at its toe, another large landslide called the North slide. In 1975, public officials were concerned that continued movement of the Manti landslide would trigger reactivation of the North slide. In response to this threat, four borings were placed in the North slide to obtain samples for testing and information